In some instances, an amplifier device can require very high input impedance. For example, a capacitive sensor feeding the amplifier device may require an impedance in the giga-ohm range. Simply inserting a very large resistor between an input of the amplifier device and a bias point to generate an appropriate operating point consumes too much silicon area to be practical.
Impedance transformations may be used to increase input impedance of the amplifier circuit. One impedance transformation technique relates to coupling a pair of transistor circuits in a current mirror arrangement with each transistor circuit of the pair having a different channel width to channel length (W/L) ratio. In some implementations, the transistor circuit having a lower W/L ratio acts as an impedance device. In these implementations, the quotient in the W/L ratios of the transistor circuits is proportional to the impedance transformation. To illustrate, when the W/L ratio of a first transistor circuit is approximately ten times larger than the W/L ratio of a second transistor circuit, the impedance value of the second transistor circuit increases by a factor of ten. However, circuit arrangements relying on differing W/L ratios in a pair of transistor circuits for impedance transformation are only easily achievable for impedances in the mega-ohm range. In order to produce impedances in the giga-ohm range for these circuit arrangements, currents on the order of picoamps may be required, making the impedance given by these circuit arrangements impractical.